The present invention relates to a control system for an automatic transmission for an automotive vehicle having an engine, and more particularly to a control system for effecting donwshift timing and engine brake running.
Commonly, automatic transmissions are automatically shiftable between different speed ratios by switching power transmission path through activation of selected one or ones of various friction elements (clutch, brake and one-way clutch). After the automatic transmission has effected a downshift, the engine speed is subject to a rapid change due to a change in speed ratio before and after the downshift. This rapid change in engine speed causes substantially great shocks on downshifting since this downshift is effected with heavy engine load (i.e., with accelerator pedal deeply depressed).
The automatic transmission uses a one-way clutch as a reaction element against a rotary member and establishes the power transmission path when the one-way clutch is activated. The one-way clutch is released to interrupt the transmission of reverse torque which the rotary member is subject to, thus preventing the occurrence of shocks and noises that go bump. However, this means that the one-way clutch will not provide a reaction against the rotary member even if the engine brake running is demanded, thus failing to effect the engine brake running even if so desired.
In order to deal with the former problem, there is effected a timing control such as to delay the commencement of a downshift operation until the engine speed reaches a value required for preventing the occurrence of substantial shocks during the shifting operation, providing a so-called neutral interval of the appropriate length. In order to deal with the latter problem, there is provided an engine brake effecting friction element as arranged in parallel to the above mentioned one-way clutch and engine running results as desired whenever the engine brake effecting friction element is activated.
According to the state of the art, as implemented in a 3-2 downshift control and an overrunning clutch activation control employed by a THM-700 type automatic transmission manufactured by General Motors in the United States or as implemented in a 3-2 downshift control and a low & reverse brake activation control employed by a RN4F02A type (or RL4F02A type) automatic transaxle manufactured by Nissan Motor Company in Japan, the former control is effected by a governor pressure variable corresponding to the vehicle speed and the latter control is effected by switching a manual selector valve.
However, according to the teaching of the above mentioned control arrangements, a governor pressure is still needed for effecting the timing control of the above mentioned downshift operation even though the governor pressure is no longer necessary for causing the automatic shift in the transmission as a result of realizing electronics control for one of purposes of eliminating the use of a governor pressure valve designed to produce the governor pressure. Since it is initiated on switching the manual selector valve, the engine brake control has little design freedom in implementing it in electronics control. Upon implementing these two controls electronically, therefore, it is necessary to provide one solenoid which is operative responsive to vehicle speed information in order to produce the governor pressure that is used for the downshift control, and it is also necessary to provide another solenoid which is operative responsive to switching the manual selector valve in order to produce an engine brake command pressure that is used for the engine brake control. However, the use of two solenoids for the respective controls is disadvantageous in view of space and cost necessary for their installation.
The present invention is based on the recognition of the following facts: (a) that an engine brake effecting friction element is arranged in parallel to a one-way clutch as previously mentioned and it does not contribute to power transmission during power-on running, so that the engine brake effecting friction element may be held deactivated during power-on running; (b) that the timing control of downshift is required on downshifting with heavy engine load; (c) that of said both controls the engine brake control is needed during coasting of the vehicle, and the downshift timing control is needed during power-on running, that is, both of these controls have different operating ranges where they are required to be effected and thus may be carried out by the common electromagnetic valve.